The present invention relates to a process for heat treatment in continuous flow of a product mixture consisting of a liquid containing solid particulates, in which the mixture is heated to a certain desired temperature in one or more heat exchange sections, is held at this temperature in a heat-holding section for a certain time, and is then cooled to the desired end temperature in one or more heat exchange sections.
The invention also relates to a device for carrying out the above process.
During the heat treatment of a product mixture consisting of a liquid containing solid particulates, for example a sterilization or pasteurization process for pumpable foodstuffs, it is generally required that each fraction of the mixture acquires at least a fixed minimum processing value. If it is ensured during the heat treatment that the fraction of the largest solid particulates acquires the minimum processing value, as a result of different heating-through times of the various fractions, the fractions of smaller solid particulates and the liquid acquires a considerably higher processing value than the minimum required processing value, which is found in a number of cases to have an adverse effect on the quality of the total product on completion of the heat treatment. For example, undesirable colour and flavour changes may have occured.
An example of such a heat treatment is the sterilization process which is carried out on a soup made of a liquid of low to medium viscosity containing solid vegetable and meat particulates. After the soup has been dispensed into cans and the cans have been hermetically sealed, the cans are then subjected with their contents to the heat treatment in a sterilizer. In particular if the cans are rotated during the sterilization process, the liquid is heated up in a short time and very soon reaches the temperature of the heating medium (steam or hot water at, say, 121.degree. C.) in the sterilizer. The outside of the solid particulates also quickly follows the temperature of the liquid. However, the heat transfer to the thermal centre of the solid particulates can take place only through conduction. The temperature of the thermal centre consequently rises considerably more slowly, and it takes quite a long time for this thermal centre to attain the minimum required processing value. The larger the solid particulates, the longer the heat treatment has to last. In such a heat treatment the smaller solid particulates and in particular the liquid are given a much higher processing value than would in fact be necessary to achieve the desired effect, for example adequate microbiological inactivation.
The same phenomenon occurs if a pumpable carrier liquid containing larger and smaller solid particulates is conveyed in a continuous stream successively through a number of first heat exchange sections, in which at least the carrier liquid is heated to a desired temperature, a heat-holding section, and a number of second heat exchange sections which are connected thereto and in which the product is cooled to a desired end temperature. Between the heating and cooling sections the carrier liquid is held in the heat-holding section at the temperature level reached in the heating sections for so long that the temperature of the thermal centre of the solid particulates carried along develops in such a way that the minimum required processing value is attained also in the thermal centre of the solid particulates. The cooling of the carrier liquid may not be started until then. The cooling could possibly be started slightly earlier if the inertia of the heat transfer to the thermal centre of the solid particulates is anticipated, in the sense that the outside of a solid particulate may already be cooled, since the temperature still continues to rise in the thermal centre for a short time. An additional factor in the above-described heat treatment is that the conveyance of solid particulates in a carrier liquid depends greatly on the Reynolds number of the carrier liquid and the dimensioning of the types of heat exchanger chosen (for example plate-type, tubular or Votator heat exchangers). Moreover, it is difficult to forecast the conveyance of the solid particulates, so that establishing the heating and cooling curves of the thermal centre of the solid particulates and thus the processing value attained is extremely unreliable. In order to be able to carry out the heat treatment process safely, the necessary safety margins must be observed in the heat treatment.